A scute () or scutum (Latin: scutum; plural: scuta "shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of , and the feet of birds. The term is also used to describe the anterior portion of the mesothorax in as well as some arachnids (e.g., the family Ixodidae, the scale ticks).
Properties
Scutes are similar to scales and serve the same function. Unlike the scales of lizards and snakes, which are formed from the epidermis, scutes are formed in the lower vascular layer of the skin and the epidermal element is only the top surface . Forming in the living
dermis, the scutes produce a horny outer layer that is superficially similar to that of scales. Scutes will usually not overlap as
snake scales (but see the
pangolin). The outer
keratin layer is shed piecemeal, and not in one continuous layer of skin as seen in snakes or lizards. The dermal base may contain
bone and produce dermal armour. Scutes with a bony base are properly called
. Dermal scutes are also found in the feet of
birds and tails of some
mammals, and are believed to be the primitive form of dermal armour in reptiles.
The term is also used to describe the heavy armour of the armadillo and the extinct Glyptodon, and is occasionally used as an alternative to scales in describing snake scales or certain fishes, such as , shad, herring, and menhaden.
Mammals
Prehistoric ancestors of mammals, the
synapsids, are thought to have scutes which were later reduced and replaced by hair. Excluding the attachment surface of
, armoured scutes or scales are almost never seen in modern
mammals. The horny scales of
are only rarely called scutes, but "scute" is used to describe the heavy armour of the
armadillo.
Turtles
The
Turtle shell is covered by scutes formed mostly of
keratin. They are built similarly to horn, beak, or nail in other species.
Fish
Some fish, such as
pineconefish, are completely or partially covered in large armored scales commonly termed scutes.
Many
Clupeomorpha have an abdominal row of scutes, which are scales with raised, sharp points that are used for protection, and in some cases they also possess dorsal scutes anterior to the dorsal fin, as in
Knightia spp.,
Diplomystus spp. and certain extant taxa.
Jacks of family
Carangidae often have a row of scutes following the
lateral line on either side.
spp. have scutes suborbitally from their
preoperculum extending anteriorly onto a triangular snout.
have five rows of scutes—modified
—and are otherwise scaleless.
Birds
The
tarsometatarsus and toes of most birds are covered in two types of scales. Large scutes run along the dorsal side of the tarsometatarsus and toes, whereas smaller scutellae run along the sides. Both structures share histochemical homology with reptilian scales; however, work on their evolutionary development has revealed that the scales in bird feet have secondarily evolved via suppression of the feather-building genetic program.
[Sawyer, R.H., Knapp, L.W. 2003. Avian Skin Development and the Evolutionary Origin of Feathers. J. Exp. Zool (Mol Dev Evol) 298B:57–72.] Unblocking the feather suppression program results in feathers growing in place of scales along the tarsometatarsus and toes.
Dinosaur species very close to the origin of birds have been shown to have had "hind wings" made of feathers growing from these areas, suggesting that the acquisition of feathers in dinosaurs was a whole-body event.
The bottoms of bird feet are covered in small, keeled scale-like structures known as reticulae. Evolutionary developmental studies on these scale-like structures have revealed that they are composed entirely of alpha keratin (true epidermal scales are composed of a mix of alpha and beta keratin).
These data have led some researchers to suggest that reticulae are in fact highly truncated feathers.
Insects and other arthropods
The term "scutum" is also used in
insect and
arachnid anatomy, as an alternative name for the anterior portion of the
mesonotum (and, technically, the
metanotum, though rarely applied in that context).
In the
, the
Ixodidae, the scutum is a rigid,
Sclerotin plate on the anterior dorsal surface, just posterior to the head. In species with eyes, the eyes are on the margin of the scutum. The flexible
exoskeleton posterior to the rigid scutum of the female tick is called the
alloscutum, the region that stretches to accommodate the blood with which the mature female tick becomes . Males do not engorge nearly as drastically as females, so they do not need a flexible alloscutum; instead the rigid scutum covers practically the entire dorsal surface posterior to the head, and may be referred to specifically as the
conscutum.
In some species of Opiliones, fused abdominal segments are referred to as a scutum.
See also
